Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

.

734 USER COMMENTS

I have a cement slab on ground floor in a concrete block home. The home has settled on two sides of the home, with a crack running through the center of the home. I was going to level the floor by adding 2 3/4'' thick concrete against the settled walls and feather to zero thickness at the center of the home where it is level. The concrete company said that any area less than 1'' thick will crack and separate from the original slab. They said there is a type of concrete that can be used in thin applications but because I need to have 2 3/4'' thick against the two walls, the thin application won't work for an area requiring more than a 1/2 inch. They therefore suggested that I take the highest point of the slab and go 1 inch thick at that point and do the whole slab. If I do this, I will be adding 3 3/4'' at the walls where the house is settled and I think that may cause too much weight and possibly make the house settle more. What would you recommend I do?
- Steve
Thursday, January 19, 2017 at 10:19 AM

I need to replace a sidewalk around my house because it slopes back to the slab instead of away. There are foundation repair pillars in some of the sidewalk 'sections', that also slope toward the slab slightly. How do I put full thickness sidewalk sections but still have a 1" to 1 1/2" layer over the repair sections? Or can I bust up the pillars on top a bunch and pour complete sidewalk sections over the broken sections? Thanks
- Wes
Sunday, January 15, 2017 at 10:14 AM

Roland,you would have to start off by cleaning the area to be pitched. While the area is damp put a 2" down to a 1/4" layer of Fast Setting Cement Patcher. Let that layer set for 30 minutes, then apply a layer of Top N Bond from a 1/2" down to a featheredge. Yon can walk on the area after a day.
- Lee-Technical Service
Tuesday, January 3, 2017 at 1:34 PM

Patrick, you can use Top N Bond applied in 1/2" layers. This material can be applied from 1/2" down to a featheredge. Give each layer at least an hour between layers.
- Lee-Technical Service
Tuesday, January 3, 2017 at 1:17 PM

The last step (stringer) to a deck is set on an existing concrete sidewalk. Problem is the riser of that last step is 2 1/2 inches too high because of sloping ground. I want to add concrete beginning at the riser (2 1/2 inches up) tapering to nothing on the sidewalk. What product should I use? How far out should I go with the tapered add-on? Thank you.
- Roland
Monday, January 2, 2017 at 7:21 AM

We removed a machine from our factory and now we have 8' X 6' by 1 1/2" deep depression in our concrete floor. What should we use to fill this in and level it out?
- Patrick
Friday, December 16, 2016 at 12:29 PM

Tony, you can use the Sakrete Polyurethane Construction Adhesive to adhere those pieces back together.
- Lee-Technical Service
Monday, December 12, 2016 at 3:06 PM

I have a concrete checker board table with a marble checker board. It sits on a concrete pedestal and has broken away. How can I secure the top to the pedestal? It would be concrete to concrete.
- Tony
Monday, December 12, 2016 at 11:11 AM

Chris, you would do better to add a French drain. Adding a curb may not solve the problem because concrete is porous. It could redirect the water to some degree but I am not sure if that would be enough. Adding a drain would help to collect and redirect the flow of water away from the wall.
- Lee-Technical Service
Friday, December 9, 2016 at 2:11 PM

I need to pour a curb on top of an existing slab. We poured a new parking lot and there is now water coming in under a wall. Any tips on product to use or methods would be a great help. The slab is 4 inches thick and 8 years old.
- Chris
Friday, December 9, 2016 at 10:16 AM

LEAVE A COMMENT

Name:

Enter Text From the Image Above: